A micromachined device typically includes a substrate and a movable component suspended relative to the substrate. The substrate and movable component face each other across a gap, and have dimensions that are large relative to the gap.
In normal operation, the substrate and movable component do not come into contact. However, if the moveable component approaches the substrate, the opposing (or “facing”) surfaces may adhere to one another, in a phenomenon commonly known as “stiction.”
Stiction is a dominant failure mechanism in micromachined devices, and can arise in a variety of ways. Stiction may arise, for example, when interfacial forces between two opposing faces of a micromachined device exceed the restoring forces of the suspension system. The stiction forces may include capillary forces, chemical bonding, electrostatic forces, and van der Waals forces.
Stiction has been addressed by providing standoffs on a surface of a MEMS device, such as in U.S. Pat. No. 5,662,771 for example, or by coating a surface of a MEMS device with a coating, such as in U.S. Pat. No. 7,364,942, for example.
The reliability of MEMS devices may also suffer from corrosion or contamination. Corrosion or contamination may occur if the MEMS device is exposed to the ambient environment, such as a humid environment for example, as might happen if the packaging or encapsulation of the device fails. Indeed, some MEMS devices are exposed to their environment in the normal course of their operation. For example, microphones and pressure sensors may be exposed to the ambient atmosphere. Particles or other contaminants may enter the MEMS device and degrade device performance, or even cause device failure.